The primary goal of this project is now limited to increasing our understanding of amino acid transport in individual nephron segments of the renal medulla by determining the mechanisms involved in amino acid transport and apparent recycling among loops of Henle and vasa recta. The title of the project has been changed to reflect this more specific goal. The planned studies will take advantage of technical improvements permitting study both in vivo and in vitro of equivalent segments of loops of Henle from Munich-Wistar rats. Determination of mechanisms involved in amino acid transport and apparent recycling in the mammalian medulla will involve: 1) in vivo continuous microinfusions of papillary loops of Henle and vasa recta; 2) in vitro microperfusions of descending and ascending thin limbs and ascending thick limbs of loops of Henle of juxtamedullary nephrons; and 3) in vitro microperfusions of descending and ascending limbs of loops of Henle of superficial cortical nephrons. These studies will involve measurements of fluxes of radiolabeled amino acids and chemical analyses of amino acids. They will examine factors regulating the fluxes. The results will contribute to the understanding of the role of amino acids in medullary function.